This proposal requests funds for an automated, single-crystal X-ray diffractometer to be used by a group of investigators to study the molecular structures of biological compounds. The diffractometer will be used for structural studies of proteins and of small biological molecules and complexes. Small-molecule systems now under investigation include calcium complexes and peptides related to elastin. Proteins being investigated by the user group include neurotoxins from scorpion venom and from sea anemones; calmodulin from rat testes; a mitogenic lectin isolated from the garden pea; human erythrocytic purine nucleoside phosphorylase; bovine ubiquitin; human serum transferrin; and small calcium-binding proteins from bone, dentin and saliva. In addition, the diffractomedter will serve as a service facility for members of the U.A.B. Cancer Center who require crystallographic studies.